The present invention relates generally to the encoding of data strings, here the encoding of sequential binary data, for digital transmission or recording systems. In particular, the novel coding technique employs high-rate quaternary codes that offer advantageous properties for transmission over metallic cables or optical fibers and for recording in storage media. Further, the present invention is directed to an encoder utilizing the novel coding technique and to new codes, and to a transmission or recording system employing the above method of encoding and the new codes.
In the prior art, many digital transmission or recording systems require the use of codes which impose restrictions on the channel input sequences. Such codes, often termed constrained, modulation, or channel codes, are mainly used to improve timing and gain control in the receiver (or decoder in a recording system), to reduce intersymbol interference, and to shape the spectrum of the transmitted or recorded sequences so that the spectrum matches the frequency characteristics of the channel. In particular, many line codes were designed to suppress the spectral components of encoded sequences near the zero frequency. Such codes are instrumental in reducing the effects of baseline wander in a receiver.
Codes of that kind are DC-free, i.e., have a first order spectral null at zero frequency, if, and only if, the running digital sum (RDS) is bounded. The number of values N that the RDS can assume determines the low frequency content of the code spectrum. For example, the modified version of the ternary MS43 line code described by P. A. Franaszek in "Sequence-State Coding for Digital Transmission", Bell System Technical Journal, Vol. 47, pp. 143-157, January 1968, which was adopted by the German Bundespost as a standard for an ISDN transceiver, is based on a bounded RDS constraint with N=6.
Binary and multilevel codes with a spectral null at zero frequency possess desirable distance properties in addition to their spectrum shaping properties and can therefore be used to improve the reliability of transmission over noisy channels. In particular, they can be used to increase the Euclidian distance at the output of partial-response channels which are often encountered in digital recording and wire transmissions.
Binary Codes for partial-response channels that expand bandwidth by using convolutional codes with good Hamming Distance properties have been considered, e.g., by J. K. Wolf and G. Ungerboeck in "Trellis Codes for Partial Response Channels", IEEE Trans. Commun., Vol. COM-34, pp. 765-773, August 1986. Binary codes that expand bandwidth by enforcing a spectral null have been given, e.g., in U.S. Pat. No. 4,888,775 by R. Karabed and P. H. Siegel, issued in 1989. All these coding schemes are binary, and are well suited for applications such as saturation recording.
Multilevel coding schemes, as discussed, e.g., by G. D. Forney and A. R. Calderbank in "Coset Codes for Partial Response Channels; or Coset Codes with Spectral Nulls", IEEE Trans. Inform. Theory, Vol. IT-35, pp. 925-943, September 1989, achieve higher immunity against noise by expanding the signal alphabet and using trellis or known coset codes adapted for partial-response channels. Low rate (R less than or equal to 1 bit/symbol) quaternary trellis codes for partial response channels have also been constructed using concatenated coding schemes and set partitioning of the channel output signal set. Multilevel coding schemes are well suited for applications such as bandwidth efficient digital transmission and AC-bias recording.